Important considerations for buying telescope lenses include aperture size, magnification, portability, ease of use, and cost. There are three main types of telescopes: reflecting telescopes with one lens and a mirror, refracting telescopes with a large objective lens and an eyepiece lens, and catadioptric telescopes, which are a hybrid of reflecting and refracting telescopes.
A telescope works by gathering light and focusing it onto the pupil of the eye. In a refracting telescope, light enters through the objective lens, which then bends that light into focus. The eyepiece lens magnifies the light collected by the objective lens so the image is visible to the unaided eye.
Because the quality of the image depends on the amount of light gathered, a larger aperture size (determined by the diameter of the objective lens) creates superior images. However, larger apertures present handling and mobility challenges, require a longer casing to focus and are more subject to damage.
The eyepiece lens magnifies or spreads out the light brought to focus by the objective lens. The magnifying power of the eyepiece is determined by the focal length of the telescope divided by the focal length of the eyepiece lens, with smaller eyepiece focal lengths resulting in increased magnification.